Gain-guided type laser diode

Abstract: A gain-guided type AℓGaInP visible light laser diode in accordance with the present invention has a characteristic in that, an n-type current-blocking layer for the current constriction in the double-heterojunction structure is highly doped with n-type impurity so as to prevent the formation of p-type inversion layer therein. When an n-GaAs current-blocking layer is formed on a p-GaInP etching stopper layer doped with zinc, the n-type impurity concentration of the n-GaAs current blocking layer is selected to be 3 x 10¹⁸ cm⁻³ or more to achieve a large kink-light output at a low oscillation threshold current.

Description: BACKGROUND OF THE INVENTION The present invention relates to gain-guided type laser diodes, and more particularly to AℓGaInP visible light laser diodes with oscillation wavelength not greater than 680 nm. The gain-guided type laser diodes are attractive light sources for barcode readers in POS and FA systems. A conventional gain-guided type AℓGaInP visible light laser diode is shown in Fig. 1 as reported in ELECTRONICS LETTERS 16th January 1987 Vol. 23, No. 2, p.85. Referring to Fig. 1, an n-GaAs buffer layer 2 is formed on an n-GaAs substrate 1. On top of the buffer layer 2 there is formed a double heterojunction structure consisting of an n-AℓGaInP cladding layer 3, a GaInP active layer 4, a p-AℓGaInP cladding layer 5, a p-GaInP etching stopper layer 6, a pair of n-GaAs current blocking layers 7 and a p-GaAs contact layer 8. Here, the fabrication step of the conventional structure shown in Fig. 1 will be described. First, a six-layer structure from the n-GaAs buffer layer 2 to the n-GaAs current blocking layer 7 is grown sequentially by using a first vapor phase growth technique such as MOVPE (Metal-Organic Vapour Phase Epitaxy). Subsequently, a trench stripe 9 is formed in n-GaAs current blocking layer 7 to expose a stripe region of p-GaInP etching stopping layer 6. Over the entire surface of n-GaAs current blocking layer 7 including stripe 9 is formed by the second MOVPE growth a p-GaAs contact layer 8 over the entire surface of which in turn a p-side ohmic electrode 10 of Ti/Pt/An is deposited. Further an n-side ohmic electrode 11 of AuGeNi is deposited on the entire under-surface of substrate 1. In this prior structure, the current is constructed by the p-GaAs contact layer 8 and the GaAs current blocking layer 7. The p-GaInP etching stopper layer 6 serves, for forming a stripe 9 in n-GaAs of current blocking layer 7 by chemical etching, to stop the etching to prevent from being over-etched, and has another purpose of reducing electrical resistance between the p-AℓGaInP cladding layer 5 and the p-GaAs contact layer 8. In such a gain-guided type laser diode, injected carriers causes a refractive index difference in the active layer to guide a light along with the stripe direction. Although the above stated gain-guided type laser diodes can be made easily compared with a buried hetero structure type laser diodes, a so called kink appears in its current-light output characteristic when the output is increased, as shown in Figure 4. This is because the active layer of the gain-guide type laser diodes has no waveguide mechanism of refractive index difference in a direction parallel to the pn junction plane. When the output is increased, a transverse mode of the output light becomes unstable and causes the kink. The conventional gain-guided type laser diodes shown in Fig. 1 causes the kink to occur at a low output light level. When the gain-guided type laser diode is used as a light source of a barcode reader, the position of the kink level is significant. The light-output level must be kept below the kink level, otherwise the barcode reader will cause a missed reading. To this end, it is desired to increase the kink level to as high a value as possible. It is known that the kink level can be increased by narrowing the stripe width of the laser diode. But when the stripe width is decreased, the threshold current is drastically increased. In view of the threshold current, the stripe width should be selected to be about 7 µm. Despite such a suitable stripe width, the kink level of the conventional gain-guided type laser diode is still low, at about 3 mW as shown in Fig. 4. EP-A-0 328 134 describes a semiconductor laser device comprising a double heterostructure formed on a GaAs substrate, comprised of In x Ga y Al 1-x-y P including an n-type cladding layer and a p-type cladding layer, one of the n-type cladding layer and the p-type cladding layer being Si-doped. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a gain-guided type laser diode capable of accomplishing laser oscillation with a high level kink in its light output, and with a suitable stripe width to achieve a low threshold current. According to one aspect of the invention there is provided a gain-guided type laser diode comprising: a first cladding layer formed on a substrate and having a first conductivity type; an active layer formed on said first cladding layer; a second cladding layer of a second conductivity type formed on said active layer; an etching stopper layer made of GaInP and of said second conductivity type formed on said second cladding layer; a current blocking layer made of GaAs and of said first conductivity type formed on said etching stopper layer with an opening exposing a stripe region of said etching stopper layer; a contact layer of said second conductivity type formed on said current blocking layer and said stripe region of said etching stopper layer; and a pair of electrodes provided on said contact layer and on the bottom of said substrate, characterized in that said etching stopper layer is doped with zinc and said current blocking layer is doped with silicon, the concentration of said silicon being in the range from 4x10 18 cm -3 to 1x10 19 cm -3 , such that the current blocking layer has an impurity concentration larger than that of said etching stopper layer so as not to form an inverted layer of said second conductivity type adjacent to said etching stopper layer within the current blocking layer. The concentration is preferably up to 9 x 10 18 cm -3 . In a preferred form of the invention the substrate is made of GaAs and a buffer layer of GaAs is formed on said substrate and having said first conductivity type; said first cladding layer is made of AlGaInP formed on said buffer layer; said second cladding layer is made of AlGaInP; and said contact layer is made of GaAs. Advantageously, the zinc concentration is about 2 x 10 18 cm -3 . BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a prior art gain-guided type laser diode. Fig. 2 is an explanatory view of a gain-guided type laser diode for analysing the problem of the prior art structure. Fig. 3 is an illustrative cross-section of a gain-guided laser diode according to the embodiment of the present invention. Fig. 4 is a graph of the laser oscillation characteristic (relationship between the forward current and light output) illustrating the effects of the embodiment in comparison with the prior art. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Prior to showing the embodiment of the present invention, detailed analysis of the prior art structure will be described with reference to analytical Fig. 2. So far, there is no specific report about the reason why the conventional gain-guided type laser diode has such low kink level despite having a narrow stripe width. The present inventor examined the prior art structure and found that a pn junction is formed within the current blocking layer closer to the etching stopper layer. That pn junction is formed by a p-type inversion layer caused by the Zn diffusion from the p-GaInP etching stopper layer. In view of crystal quality, doped impurity concentration in both of the n-GaAs current blocking layer and p-GaInP etching stopper layer has been selected to be the same level such as 1 x 10 18 cm -3 . During the second MOVPE growth of p-GaAs contact layer 8, the thermal treatment cycle causes thermal diffusion of Zn impurity doped in p-GaInP etching stopper layer 6 into n-GaAs current blocking layer 7. In general it is thought that, taking the boundary conditions into consideration, solid-solid diffusion concentration of Zn from etching stopper layer 6 into current blocking layer 7 can be reduced by about one order of magnitude to up to 10 17 cm -3 in Zn concentration, which is estimated to be one order of magnitude smaller than the doped-Si concentration in current blocking layer 7 (1 x 10 18 cm -3 ), thus remaining unimportant. It however, has been demonstrated experimentally that after practical growth of p-GaAs contact layer 8, the pn junction (shown by a dashed line in Fig. 2) was formed by Zn diffusion in n-GaAs current blocking layer 7. Under growth conditions of temperature 650°C for 30 minutes,Zn diffusion was observed down to 0.1 µm deep in n-GaAs current blocking layer 7. It is believed that an excessive amount of electrically-inert Zn impurity from the doped Zn impurity (concentration: 1 x 10 18 cm -3 ) existing in p-GaInP etching stopper layer 6 enhances the Zn diffusion into the overlying n-GaAs current blocking layer 7 caused by the thermal treatment. As shown in Fig. 2 therefore a p-GaAs diffused inversion layer 12 is formed in the n-GaAs current blocking layer. It serves as a path allowing a part of the injected current necessary for laser oscillation to flow therethrough as a leakage current (indicated by i 1 in Fig. 2), with a result that the effective current-injection width is more than the stripe width, which in turn leads to a reduced kink light output as shown in the current-light output characteristic of Fig. 4, and a higher oscillation threshold current. With a stripe width of 7 µm, a kink in the light output appears at 3 mW. Referring to Fig. 3, an embodiment of the present invention will be described below. On an n-GaAs substrate 13 with major plane (100) (n-type impurity concentration: 2 x 10 18 cm -3 ) a double heterojunction structure is grown in the following: by MOVPE technique at reduced pressures using organic III-group metal compounds (trimethyl indium, triethyl gallium, trimethyl aluminium) and V-group element hydrides (PH 3 , AsH 3 ) as starting materials, an n-GaAs buffer layer (n-type impurity concentration: 1 x 10 18 cm -3 ) 14 of 0.5 µm thick, an n-(Al 0.6 Ga 0.4 ) 0.5 In 0.5 P cladding layer (n-type impurity concentration: 5 x 10 17 cm -3 ) 15 of 1 µm thick, a Ga 0.5 In 0.5 P active layer 16 of 0.06 µm thick, a p-(Al 0.6 Ga 0.4 ) 0.5 In 0.5 P cladding layer (p-type impurity concentration: 3 x 10 17 cm -3 ) 17 of 1 µm thick, a p-Ga 0.5 In 0.5 P etching stopper layer (p-type impurity concentration: 2 x 10 18 cm -3 ) 18 of 0.1 µm thick, and an n + -GaAs current blocking layer (n-type impurity concentration: 5 x 10 18 cm -3 ) 19 of 0.6 µm are shown in sequence one on top of another. Subsequently using SiO 2 film or photoresist film as mask, a trench stripe 20 is formed which extends through n + -GaAs current blocking layer 19 to expose p-Ga 0.5 In 0.5 P etching stopper layer 18. This is obtained by selective etching of n + -GaAs current blocking layer 19 and nothing else by using a liquid mixture of H 3 PO 4 , H 2 O 2 and H 2 O. Subsequently a p-GaAs contact layer 21 (p-type impurity concentration: 3 x 10 18 cm -3 ) is grown to a thickness of 4 µm by the second MOVPE using triethyl gallium and AsH 3 as starting materials under a reduced pressure as shown in Fig. 3. Thereafter a p-side electrode 22 of Ti/Pt/An is formed on p-GaAs contact layer 21 and a n-side electrode 23 of AuGeNi on the undersurface of substrate 13. Thus a semiconductor laser of the structure shown in Fig. 3 is completed. N + -GaAs current blocking layer 19 is doped with an n-type impurity of Si from SiH 4 used as a starting material to a concentration of 5 x 10 18 cm -3 , and thereby the prevention of diffusion of Zn into n + -GaAs current blocking layer 19 is achieved. In the foregoing embodiment, although the n-type impurity (Si) concentration is selected to be 5 x 10 18 cm -3 , experiments shows that a suitable concentration is ranged from 4 x 10 18 cm -3 to 9 x 10 18 cm -3 as shown in the TABLE 1 to show the high kink level of around 6 mW (5.8 mW to 6.2 mW) with low threshold current level of around 60 mA (58 mA to 60 mA). Sample n-type impurity concentration Threshold Current Kink light output 1 4 x 10 18 cm -3 58 mA 6.0 mW 2 5 x 10 18 cm -3 60 mA 5.8 mW 3 9 x 10 18 cm -3 60 mA 6.2 mW The lower limit of the n-type impurity concentration of the GaAs current blocking layer is related to the excessive amount of Zn impurity in the p-GaInP etching stopper layer. Generally, since the upper limit of the Zn doping concentration in a GaInP layer is 2 x 10 18 cm -3 due to saturation phenomenon peculiar to the material. The foregoing experiments indicates that the formation of the p-type inversion layer is prevented by selecting the impurity concentration of the n + -GaAs current blocking layer to be 4 x 10 18 cm -3 or more. The upper limit of the n-type impurity concentration of GaAs current blocking layer may be 1 x 10 19 cm -3 . If exceeding this upper limit, the crystal quality of n + -GaAs current blocking layer will be deteriorated and results in shortening the life time of the laser diode. In view of the product yield, it is proper to select its impurity concentration to be 9 x 10 18 cm -3 or less. As described above, the impurity concentration of the current blocking layer of the present invention should be selected to be between 4 x 10 18 cm -3 to 1 x 10 19 cm -3 . The most preferable range is 4 x 10 18 cm -3 to 9 x 10 18 cm -3 . In other words, impurity concentration of the current blocking layer is selected to be about twice or more of that of the etching stopper layer. A suitable stripe width is not restricted to 7 µm width as in the case of the foregoing embodiment. However, if the stripe width exceeds more than 8 µm, the kink level is drastically decreased while the stripe width is selected to be narrower than 6 µm, the threshold current increases drastically. Therefore, it is preferable to select the stripe width in the range of 6 µm to 8 µm in the present invention. According to the present invention, since the kink level is increased to about 6 mW, a barcode reader employing the present invention as a light source can operate without causing miss-reading when it is used at output power of 5 mW. Furthermore, the barcode reader of high output power such as 5 mW can be used for reading an object located at a long distance point such as about 2 m away. This is in contrast to a conventional barcode reader of low output power of 3 mW, which can be used only a short distance such as 30 cm or less. This is an example of a benefit attainable with the described embodiment of the present invention. As described above, the present invention, because of lack of a p-type inversion layer formed by the Zn diffusion into the current blocking layer 19, enables the realisation of a laser diode with the kink in light output at a higher level than hitherto and at a low threshold current as compared with the prior art gain-guided type laser diode.

Claims: A gain-guided type laser diode comprising: a first cladding layer (15) formed on a substrate (13) and having a first conductivity type; an active layer (16) formed on said first cladding layer (15); a second cladding layer (17) of a second conductivity type formed on said active layer (16); an etching stopper layer (18) made of GaInP and of said second conductivity type formed on said second cladding layer (17); a current blocking layer (19) made of GaAs and of said first conductivity type formed on said etching stopper layer (18) with an opening exposing a stripe region of said etching stopper layer (18); a contact layer (21) of said second conductivity type formed on said current blocking layer (19) and said stripe region of said etching stopper layer (18); and a pair of electrodes (22,23) provided on said contact layer (21) and on the bottom of said substrate (13), characterized in that said etching stopper layer (18) is doped with zinc and said current blocking layer (19) is doped with silicon, the concentration of said silicon being in the range from 4x10 18 cm -3 to 1x10 19 cm -3 , such that the current blocking layer has an impurity concentration larger than that of said etching stopper layer so as not to form an inverted layer of said second conductivity type adjacent to said etching stopper layer within the current blocking layer. A gain-guided type laser diode as claimed in Claim 1, wherein said zinc concentration is about 2 x 10 18 cm -3 . A gain-guided laser diode as claimed in Claim 1 or 2 which the silicon concentration is up to 9 x 10 18 cm -3 . A gain-guided type laser diode as claimed in Claim 1, wherein said substrate (13) is made of GaAs and a buffer layer (14) of GaAs is formed on said substrate and having said first conductivity type; said first cladding layer (15) is made of AlGaInP formed on said buffer layer (14); said second cladding layer (17) is made of AlGaInP; and said contact layer (21) is made of GaAs.

Inventors: KAWANO HIDEO, KAWANO, HIDEO, KAWANO, HIDEO, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: H01S3
IPC codes: H01S5